Stanford University Ph.D. Oral Examination
Xuefeng (Steve) Zhuang
Department of Electrical Engineering
Advisor: Professor Butrus T. Khuri-Yakub
Date: Tuesday, February 19, 2008
Time: 2 PM (Refreshments served at 1:45 PM)
Location: Packard room 202
Title: Capacitive Micromachined Ultrasonic Transducers (CMUTs) with
Through-Wafer Interconnects
Abstract:
Capacitive micromachined ultrasonic transducer (CMUT) technology is a
promising candidate for making 2D ultrasound transducer arrays for
applications such as 3D medical ultrasound, non-destructive evaluation
and chemical sensing. Advantages of CMUTs over traditional
piezoelectric transducers include low-cost batch fabrication, wide
bandwidth, and ability to fabricate arrays with broad operation
frequency range and different geometric configurations on a single
wafer. When incorporated with through-wafer interconnects, a CMUT
array can be directly integrated with front-end IC to mitigate the
effects of the parasitic capacitance from the connection cables.
Through-wafer via is the existing interconnect scheme for 2D CMUT
arrays, and many other types of MEMS devices. However, efforts to
integrate the direct fusion bonding process with through-wafer via
interconnects have been unsuccessful. The interconnect fabrication
steps degrade the surface conditions of the wafer, making it difficult
to bond.
In this talk I present a new through-wafer interconnect technique that
is compatible with MEMS device fabrication in general, including both
the surface-micromachining and the direct fusion bonding processes.
The new technique uses through-wafer trenches to separate array
elements and utilizes the silicon substrate as the conductive
electrode. Two exemplary implementations incorporating CMUT arrays
are presented, one with a carrier wafer and the other with a built-in
silicon frame structure for mechanical support. Both implementations
reduce fabrication complexity compared to the through-wafer via
process, and result in low series resistance and small parasitic
capacitance. 2D CMUT arrays integrated with trench-isolated
interconnects show high output pressure (2.9 MPa), wide bandwidth
(95%), small pulse-echo amplitude variation (sigma = 6.6%), and
excellent yield (100%). An important added benefit of the
trench-isolated interconnect is the capability to realize flexible
arrays, which is also discussed in the presentation. I will also
demonstrate 3D ultrasound imaging based on a trench-isolated 2D array
integrated with an IC.